First report of black leaf streak disease in bananas caused by Pseudocercospora fijiensis on Mauritius island

Abstract

Pseudocercospora fijiensis, the causal agent of the black leaf streak disease of bananas (plants in the genus Musa) (BLSD), is considered to be the major economic threat to export-banana cultivation (de Bellaire, Fouré, Abadie, & Carlier, 2010). The disease has a worldwide distribution throughout the humid tropical regions and has been previously reported in the Southwestndian Ocean (SWIO) area: in 1993 in Mayotte and Comoros islands (DR Jones & Mourichon, 1993), in 2000 in Madagascar (Jones, 2003; Rivas, Zapater, Abadie, & Carlier, 2004) and in 2018 in Reunion Island (Rieux et al., 2019). In Mauritius, the presence of Pseudocercospora fijiensis was suspected in 1996 (Soomary & Benimadhu, 1998) but has never been confirmed, as symptoms could have been confounded with Pseudocercospora musae or Pseudocercospora eumusae, two causal agents of others leaf spot diseases of banana which were previously described in Mauritius in 1959 (Orieux & Felix, 1968) and 2000 (Carlier, Zapater, Lapeyre, Jones, & Mourichon, 2000), respectively. In March 2022, typical BLSD symptoms were observed at relatively low prevalence in a Cavendish crop located in the “Balance John” area (site S1 on Fig. S1-A) of Mauritius island. Typical early symptoms (stages 2) were 1- to 4-mm long brown streaks at the abaxial leaf surface, and typical older streaks (stages 3 and 4) were also observed (Fig. S1-B). These symptoms were mixed with symptoms of ELSD caused by P. eumusae. Since both species cannot be clearly distinguished only on the description of symptoms, conidial sporulation on stages 2 was checked in the laboratory (Ngando et al., 2015) since P. eumusae does not produce conidia on these young stages. In April 2022, banana leaves bearing symptoms of leaf spot diseases were collected in 7 different sites (Fig. S1-A). All leaf fragments were sent to the CIRAD laboratories where molecular diagnosis was performed following the protocol developed by Arzanlou et al. (2007). In brief, genomic DNA was extracted from ground leaf fragments displaying symptoms using the DNeasy® Plant Mini Kit (Qiagen®, Courtaboeuf, France). At each site, a total of 6 lesions cut from 6 different leaves were pooled. The DNA extracts were added as templates for real-time PCR assay designed to specifically detect the presence of P. fijiensis, P. musae and P. eumusae using MFbf/MFbrtaq/MFbp, MEbf/MEbrtaq/FMep and MMbf/Mmbrtaq/FMep primers and probes, respectively (Arzanlou et al., 2007). Both positive and negative controls were included in the assay and every sample reaction was duplicated. P. fijiensis was detected from 2 out of 7 sites (S2 and S7, see Fig.S2-B). P. eumusae was detected at all sites while P. musae was found in one site only (S6). Interestingly, our results also showed coinfection by P. fijiensis – P. eumusae & P. musae – P. eumusae on several sites. The presence of P. fijiensis was further confirmed by several investigations performed on conidia isolated from S2 samples including i) morphological observations of conidia displaying P. fijiensis type description (Pérez-Vicente, Carreel, Roussel, Carlier, & Abadie (2021), Fig. S2-A), ii) DNA sequencing of 16S ribosomal gene with ITS1 & ITS4 primers (GenBank accessions Nos. OR515818-OR515810) with BLAST results displaying percentages of identity > 99.70% with type strains and iii) Koch’s postulates were fulfilled by artificial inoculation of detached leaf pieces as described in Pérez-Vicente, Carreel, Roussel, Carlier, & Abadie (2021) (Fig. S2-D). In brief, for the artificial inoculation, symptoms obtained after inoculation of both a strain isolated in Mauritius (S2-MAU) and a positive control (T+) were compared and shown to be typical of P. fijiensis species for the 3 replicates. To the best of our knowledge, this is the first official report of P. fijiensis and BLSD in Mauritius Island. This revelation holds significant importance for both the agricultural and scientific communities, shedding light on the potential spread and impact of this devastating pathogen in previously unaffected regions. From a global perspective, this discovery underscores the interconnectedness of agricultural ecosystems and the need for vigilance in monitoring and responding to emerging plant diseases in an increasingly interconnected world (Vega et al. 2022). Future investigations will be required to monitor the spread of BLSD on the island, describe the genetic structure of populations and identify routes of invasion at the SWOI scale.